I have a Tamron 70 - 300mm lens. I have noticed that the best focus I can achieve at the 300mm setting is not as sharp as the focus I usually get at a lower setting. Is this a common problem? Is there any way to work around this? The camera I use is a Canon 1000D.

4 Answers
4

Every lens has a different sharpness. Some zooms are better at the beginning of the range, some in the middle, some at the end.
Maybe yours isn't at its best on the end.
Check online reviews, there sure be someone who noted that.

If not, some other things can be checked:

sharpness change depending on the aperture size. How is the sharpness when closing aperture two or 3 stops down? For example, going to f8 or f11 if your lens is max f4.

at 300mm and more, any move can produce a slight defocus. What speed are you shooting at? A basic rule is to use 1/zoom seconds. In your case, use a speed greater than 1/300 of a second.

back or front focus problem: if the back/front focus is not correct, you will lose focus while changing the zoom factor. If you don't know what I'm talking about, check with a pro. Most of the time you can't do anything about it on your own.

From a casual read it seems that the reviewer rated the center sharpness of the lens at 300mm as 'excellent' and the edge sharpness as 'great,' within the center range of the f-stops (f/4 to f/11). As mentioned by jrista, I would expect that the sharpness of the lens would drop off (probably considerably) as you work your way below f/4 and above f/11...

This is a common facet of zoom lenses. Unlike prime lenses, where all lens elements can be utilized to maximize the image quality at a single focal length, zoom lenses have to make trade-offs between image quality, focus, and various optical aberrations. Smaller zoom ranges usually tend to have better IQ throughout the focal range, while lenses with larger zoom ranges often have a sweet spot where IQ is at its best, which falls off as you move towards the extremes. Usually, lenses with less than 4x zoom difference between the minimum and maximum will offer better IQ, while lenses with more than 5x will usually have to compromise in one or more areas. IQ issues are usually compounded with off-brand lenses like Tamron when compared with similar brand lenses from Nikon, Canon, etc.

It should be noted that compromises are usually optical in nature, and lenses tend to experience their worst optical aberrations when shot wide open. If your lens is soft at 300mm, you can try to stop down the aperture a bit, and see if things sharpen up. You should usually be able to gain some improvement in sharpness by tightening the aperture a stop or two on most lenses. You might also want to make sure you are not simply experiencing some kind of front/back focus problem at 300mm. If your camera has any kind of focus fine tuning adjustment, you might try tweaking that a bit to see if it is even possible to get clear focus at 300mm. If so, you should either get your lens serviced, or exchange it for another copy.

Finally, it should be noted that as focal length increases, so does the effect of any amount of motion, either caused by holding or touching the lens or camera when taking a shot, or by motion of the object you are photographing. You will probably want to make sure your lens is on a tripod for best use at 100mm or beyond. If you can't stabilize the lens on your own, it might be a good idea to sell the Tamron and buy a lens with some kind of image stabilization in it, such as one of the Canon 70-200mm IS variants. IS can greatly improve the handholdability of a lens at longer focal lengths.

This is normal — lens design is an exercise in compromise, and making a lens zoom comes at a cost. That cost can be paid with size, weight, expensive exotic elements, or just plain accepting more image quality flaws.

In addition to overall sharpness decrease, lenses often back- or front-focus on a given camera body differently at different focal lengths. With a prime lens this can be compensated for (by sending your camera and lenses for service, or on higher-end cameras through custom settings), but I'm not aware of any camera which has a per-lens compensation which can have multiple settings for different focal lengths of the same zoom lens.